In the past, there have been ceramic matrices (e.g. alumina, boron carbide, and mullite) with silicon carbide whisker reinforcement such as that disclosed in U.S. Pat. No. 4,543,345 to Wei (U.S. Reissue Pat. No. 32,843 [reissued on Jan. 24, 1989] and No. 34,446 [reissued on Nov. 16, 1993] to Wei). According to the Wei patent, the incorporation of silicon carbide whiskers increased the fracture toughness of the ceramic body. The Wei patent mentions two specific kinds of silicon carbide whiskers; namely, the grade F9 (or SC-9) silicon carbide whiskers from ARCO (now Advanced Composite Materials Corporation of Greer, S.C.) and “Tokamax” silicon carbide whiskers from Tokai Carbon Company, Tokyo, Japan. The average diameter of these whiskers was 0.6 micrometers with a length of 10-80 micrometers and an average aspect ratio of 75.
There have also been ceramic cutting tools with silicon carbide whisker reinforcement. In this regard, U.S. Pat. No. 4,789,277 to Rhodes et al. entitled METHOD OF CUTTING USING SILICON CARBIDE WHISKER REINFORCED CERAMIC CUTTING TOOLS and U.S. Pat. No. 4,961,757 to Rhodes et al. for REINFORCED CERAMIC CUTTING TOOLS each disclose the use of silicon carbide whiskers (the content ranges from 2 volume percent to 40 volume percent) alumina matrix. The alumina matrix may be “doped” with up to 30% zirconia, hafnia and titanium carbide. The particular silicon carbide whiskers disclosed in these Rhodes et al. patents are silicon carbide whiskers made by the Advanced Materials group of ARCO Chemical Company (now Advanced Composite Materials Corporation of Greer, S.C.). These silicon carbide whiskers have an average diameter of about 0.6 micrometers and an aspect ratio on the order of 15-150.
PCT/US 86/00528 Patent Application to Rhodes et al. entitled HIGH DENSITY REINFORCED CERAMIC BODIES AND METHOD OF MAKING SAME has as its focus the pressureless sintering of whisker-reinforced ceramic bodies. This document mentions a whisker content of between 0.5 and 21 volume percent. The specific examples teach the use of an alumina matrix with SiC whisker contents from 6.1 volume percent to 29.2 volume percent. This document mentions that the silicon carbide whiskers have lengths equal to 10-100 micrometers and average diameters on the order of 1.0 micrometer or less. The example shows SiC whiskers with a length of 10-80 micrometers and a diameter equal to 0.6 micrometers.
U.S. Pat. No. 5,656,217 to Rogers et al. discloses pressureless sintering of ceramics reinforced with silicon carbide whiskers. The as-received silicon carbide whiskers are monocrystalline and have a diameter between about 0.4 to about 0.6 micrometers and an aspect ratio equal to 15-150. U.S. Pat. No. 5,389,586 to Rogers et al. entitled PRESSURELESS SINTERING OF WHISKER REINFORCED COMPOSITES suggests using the silicon carbide whiskers disclosed in U.S. Pat. No. 4,961,757 to Rhodes et al.
U.S. Pat. No. 5,059,564 to Mehrotra et al. for an ALUMINA-TITANIUM CARBIDE-SILICON CARBIDE COMPOSITION pertains to an alumina-based matrix containing a dispersion of SiC whiskers and a TiC phase. The SiC whiskers comprise 1.0 to less than 30 volume percent with the most preferred range being 2.5 to 20 volume percent. The silicon carbide whiskers have a diameter equal to 0.3 to 0.7 micrometers and a length of 20 to 50 micrometers. The TiC comprises 5 to 40 volume percent, and preferably, with up to 3 volume percent of a sintering aid residue.
U.S. Pat. No. 5,439,854 to Suzuki et al. pertains to a cutting tool that contains 40 weight percent or more of TiC, and 5 to 40 weight percent of silicon carbide whiskers (of a length equal to 1-20 micrometers and a diameter equal to 0.2-1.5 micrometers. The cutting tool may also contain up to 40 weight percent alumina, as well as sintering aids. Up to 40 weight percent of the TiC may be substituted by Ti or a Ti-based compound such as a nitride, boride, or oxide.
U.S. Pat. No. 5,955,390 to Mehrotra et al. (and U.S. Pat. No. 6,204,213 to Mehrotra et al.) pertains to a ceramic composite that comprises a matrix and ceramic whisker reinforcement. The examples include a titanium carbide-alumina matrix with silicon carbide whisker reinforcement, a titanium carbonitride-alumina matrix with silicon carbide whisker reinforcement, and a titanium molybdenum carbide-alumina matrix with silicon carbide whiskers. This patent discloses two kinds of silicon carbide whiskers. The first are Tokai silicon carbide whiskers that have an average length equal to 20-50 micrometers with an average diameter equal to 0.3 to 1 micrometers. The second are the SC-9 silicon carbide whiskers that had an average length equal to 10-80 micrometers and an average diameter equal to 0.6 micrometers.
U.S. Pat. No. 5,754,142 to Johnsson et al. pertains to SiC whisker reinforced alumina cutting tools. The '142 patent describes the whiskers as comprising monocrystals with a diameter of 0.2-10 micrometers and a length of 2.5-100 micrometers with a length to diameter ratio preferably of 5-30.
U.S. Pat. No. 4,956,316 to Sawyer concerns a silicon carbide whisker reinforced material. The silicon carbide whiskers used in Sawyer are single crystals containing alpha, beta and mixed alpha and beta phases of silicon carbide. The average diameter of the whiskers is about 0.6-2 micrometers and the length about 10-80 micrometers. It is preferred that the aspect ratio of the whiskers be less than about 30.
U.S. Pat. No. 4,867,761 to Brandt et al. discloses silicon carbide whiskers that comprise of monocrystals with a diameter of 0.5-10 micrometers and a length of 2.5-100 micrometers characterized thereof that the length/diameter ratio preferably is 5-10.
In the past, silicon carbide whiskers have been subjected to ball milling. In this regard, U.S. Pat. No. 5,376,600 to Tiegs teaches extensive ball milling. However, in describing prior processes, the '600 patent mentions ball milling for as short a duration as 0.5 hours to lower the size distribution. U.S. Pat. No. 5,449,647 to Brandt teaches that the aspect ratio can be controlled by ball milling.
In the past, silicon carbide whiskers have been subjected to heat treatments. For example, U.S. Pat. No. 5,017,528 to Tiegs et al. concerns the treatment of silicon carbide whiskers. This patent shows heat treating SiC whiskers in an oxygen sparaging atmosphere.
Table I set forth below presents certain physical properties of some prior art commercial cutting tools.
TABLE ISelected Physical PropertiesOf Certain Commercial Cutting ToolsCuttingHardnessVHN (GPa)KIC (E&C)ToolHRA18.5 Kg LoadMPam ½WG-30094.619.46.5-7.5HC694.619.45.1K09094.819.14.7Referring to these commercial cutting tools, the WG-300 cutting tool is sold by Greenleaf Corporation of Saegertown, Pa. and has a composition of about 25 volume percent to about 30 volume percent SiC whiskers and the balance alumina. The HC6 cutting tool is sold by NTK Cutting Tool Division of NGK Spark Plugs (USA), Inc. of Farmington Hills, Mich., and has a composition of about 70 weight percent TiC and the balance alumina. The K090 cutting tool is made by Kennametal Inc. of Latrobe, Pa. and has a composition of about 70 volume percent alumina and 30 volume percent TiC. Each of these compositions may also contain minor amounts of one or more sintering aids. Although not listed in Table I above, commercial grade CC670 is sold by Sandvik Coromant. Grade CC670 is another silicon carbide whisker-reinforced alumina cutting tool that has a composition substantially identical to the composition of WG-300.
As can be seen by a review of the above documents, the predominant type of silicon carbide whisker used as a reinforcement for ceramics matrices, and especially the ceramic matrix in a cutting tool, is a finer diameter monocrystalline silicon carbide whisker that has an average diameter of about 0.6 micrometers and an average length of about 10 to about 80 micrometers and an average aspect ratio of about 75. These finer diameter silicon carbide whiskers are more expensive than as-received coarse silicon carbide whiskers (i.e., silicon carbide whiskers that have a so-called coarse morphology). Heretofore, as-received coarse silicon carbide whiskers have not been satisfactorily employed as reinforcement in ceramic matrices, and especially in ceramic matrices of cutting tools.
Typically, coarse morphology silicon carbide whiskers have an average diameter of between about 1.2 to about 1.7 micrometers and an average aspect ratio of between about 6 and about 10. These coarse morphology silicon carbide whiskers also have a high percentage of silicon carbide clusters (also known as mat) wherein these clusters may comprise up to about 20 weight percent of the silicon carbide whiskers and have a size as high as 50 micrometers. Scanning electron microscopy (SEM) has shown that this mat is actually silicon carbide whiskers bonded together by silica. The bond between these silicon carbide whiskers and the silica is relatively strong so that techniques like ultrasonication or the use of chemical dispersants will not break up the mat. These coarse morphology silicon carbide whiskers also have a rough surface wherein the surface area (BET) may be greater than about 3 square meters per gram. These coarse morphology silicon carbide whiskers also may have a high percentage of free silica wherein the silica comprises about 3 (or possibly up to about 5) weight percent of the silicon carbide whiskers.
The presence of the silica mat is disadvantageous to the effective reinforcement of the matrix by the silicon carbide whiskers. The same is true for the high free silica content. The rough surface (i.e., high surface area) of the silicon carbide whisker also is disadvantageous to the effective reinforcement of a ceramic matrix by the as-received coarse silicon carbide whiskers. Ineffective reinforcement may be due to any one or more of low density, reduced hardness or reduced fracture toughness of the ceramic composite.
It would be desirable to provide a method to process as-received coarse silicon carbide whiskers so that these whiskers would be suitable for use as a reinforcement in ceramic matrices. More specifically, it would be desirable to provide a method that processes the as-received coarse silicon carbide whiskers so as to reduce (or even eliminate) the content of the mat in the silicon carbide whiskers. It would also be desirable to provide a method that processes the as-received coarse silicon carbide whiskers that reduces (or even eliminates) the free silica content. Finally, it would be desirable to provide a method that processes the as-received coarse silicon carbide whiskers so as to reduce the surface roughness of the silicon carbide whiskers.